The invention relates to electric heating devices. It is particularly concerned with the rapid heating of fluid media using at least one porous member. This may consist of a plurality of polycrystalline metal whiskers grown out of the gas phase, felted together and connected at their points of contact. The member is heated electrically and the medium which is to be heated is passed through it.
An electric heating device of this type is described, inter alia, in German Pat. No. 1,288,705.
Porous members which consist of inter-connected polycrystalline metal whiskers can be produced with an extremely large inner surface area for their overall volume. This is possible because the polycrystalline metal bodies have a well above average strength for their diameter (of the order of a few microns to less than 1 micron) and can therefore be worked into skeletal structures capable of standing up to fluid or gas pressures at which skeletons of other materials would be torn apart or excessively compressed. Manufacture and properties of such polycrystalline metal whiskers are described, inter alia, in German Pat. No. 1,224,934 and in the "Zeitschrift fur Metallkunde" Vol. 59 (1968, No. 1, pages 18-22.
Such porous members are used with great success as resistance heating elements for the vaporizing or atomization of liquid fuels (see Ingenieur Digest 1972, No. 12, pages 43 to 44). A certain difficulty arises, however, from the ready electrical conductivity, i.e. the low electrical resistance of such a whisker skeleton, which gives rise to heavy currents. The use of compensating resistances for the purpose of reducing the current strength is not justifiable on economic grounds, and the use of a transformer for the purpose of lowering the voltage is in many cases impossible or not desirable. Furthermore, the electrical resistance of such whisker skeletons can be varied only within relatively narrow limits.